U.S. patent number 5,280,347 [Application Number 07/782,620] was granted by the patent office on 1994-01-18 for color image sensing device.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Takashi Sasaki, Akihiko Shiraishi, Akira Suga.
United States Patent |
5,280,347 |
Shiraishi , et al. |
January 18, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Color image sensing device
Abstract
A color image sensing device comprises an image sensor having
pixels of the offset sampling structure in which the horizontal
pitch is P.sub.H, the vertical pitch is P.sub.V and the horizontal
offset amount is P.sub.H /2, and a color filter array comprising
three or four types of color filters provided in one-to-one
relation to the pixels of the image sensor. The color filter array
has the offset sampling structure in which those types of color
filters are each arranged with the horizontal pitch of 2P.sub.H,
the vertical pitch of 2P.sub.V and the horizontal offset amount of
P.sub.H.
Inventors: |
Shiraishi; Akihiko (Kawasaki,
JP), Suga; Akira (Tokyo, JP), Sasaki;
Takashi (Yokohama, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
27554464 |
Appl.
No.: |
07/782,620 |
Filed: |
October 25, 1991 |
Foreign Application Priority Data
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Oct 30, 1990 [JP] |
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2-290819 |
Oct 30, 1990 [JP] |
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2-290820 |
Nov 2, 1990 [JP] |
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2-295212 |
Nov 5, 1990 [JP] |
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2-297071 |
Nov 5, 1990 [JP] |
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2-297072 |
Nov 6, 1990 [JP] |
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2-299028 |
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Current U.S.
Class: |
348/223.1;
348/E9.01; 348/242; 348/273 |
Current CPC
Class: |
H04N
9/04515 (20180801); H04N 9/04561 (20180801); H04N
2209/046 (20130101) |
Current International
Class: |
H04N
9/04 (20060101); H04N 009/70 (); H04N 009/77 () |
Field of
Search: |
;358/44,41,43,36,37,29C,27,31,166,167 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0013191 |
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Jan 1987 |
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JP |
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0053586 |
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Mar 1987 |
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JP |
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0207089 |
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Sep 1987 |
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JP |
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Primary Examiner: Groody; James J.
Assistant Examiner: Lee; Michael H.
Attorney, Agent or Firm: Robin, Blecker, Daley &
Driscoll
Claims
What is claimed is:
1. An image signal processor comprising:
image pattern detecting means for detecting different image
patterns of an image signal;
color signal processing means for performing on the image signal
different color signal processings which are respectively adapted
for reduction of false colors caused by said different image
patterns to be detected; and
selection means for selecting one of said different color signal
processings performed by said color processing means on the basis
of a detection result of said image pattern detecting means.
2. An image signal processing according to claim 1, wherein said
color signal processing means carries out the process for
converting color signals into achromatic signals.
3. An image signal processor according to claim 1, wherein said
image pattern detecting means detects changes of an image in both
the horizontal and vertical directions for detecting a pattern of
the image.
4. An image signal processor according to claim 3, wherein said
image pattern detecting means includes a horizontal band-pass
filter and a vertical band-pass filter.
5. An image signal processor comprising:
processing means for performing low-pass filtering of a luminance
signal in different predetermined directions;
determination means for determining whether or not there are small
luminance changes in an image in said predetermined different
directions; and
selection means for selecting the low-pass filtering performed by
said processing means in the direction in which small luminance
change is determined to exist by said determination means, when
said determination means determines that a small luminance change
exists.
6. An image signal processor according to claim 5, wherein said
decision means makes a decision based on absolute values of outputs
of horizontal and vertical band-pass filters for the luminance
signal.
7. An image pickup apparatus comprising:
(a) image pickup means for converting an image pickup light from an
object into electrical signal, said image pickup means having a
plurality of color filters arranged in a predetermined order on a
surface thereof;
(b) detection means for detecting that there is a possibility that
a false color is generated, on the basis of a relation between the
arrangement of said plurality of color filters and a pattern of the
object;
(c) processing means for processing said electrical signal output
from said image pickup means with one of a plurality of
predetermined characteristics; and
(d) control means for changing said predetermined characteristic of
said processing means according to an output of said detection
means.
8. An apparatus according to claim 7, wherein said plurality of
color filters includes Ye, Cy, G and Ma filters.
9. An apparatus according to claim 7, wherein said predetermined
characteristics include a matrix coefficient for color
separation.
10. An apparatus according to claim 7, wherein said processing
means performs a processing for producing color signals of R, G and
B from the output of said image pickup means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arrangement of color filters
for an image sensor in which pixels have the offset sampling
structure.
2. Related Background Art
FIGS. 1, 2 and 3 show conventionally known examples of an
arrangement of color filters for a color solid-state image sensing
device. In FIG. 1, green light transmissive filters (hereinafter
referred to as "Gr filters") are vertically arrayed in the stripe
form. Red light transmissive filters (hereinafter referred to as
"Rd filters") and blue light transmissive filters (hereinafter
referred to as "Bl filters") are arrayed two rows apart vertically
and one column apart horizontally between the Gr filters.
In FIG. 2, magenta light transmissive filters (hereinafter referred
to as "Mg filters"), green light transmissive filters, cyan light
transmissive filters (hereinafter referred to as "Cy filters"), and
yellow light transmissive filters (hereinafter referred to as "Ye
filters") are arranged in the sequence as shown in units of eight
color filters consisted of horizontal two pixels and vertical four
pixels.
FIG. 3 is concerned with a solid-state image sensor having the
offset sampling structure disclosed in Japanese Patent Application
No. 1-24433, for example. In this offset sampling structure, Rd, Gr
and Bl filters are arranged in units of three color filters
consisted of horizontal three pixels and vertical one pixel with an
offset amount of 1.5 pixels in the horizontal direction between two
rows.
FIGS. 4, 5 and 6 are characteristic diagrams of the first quadrant
as obtained by expressing color light carriers in the color filter
arrangements of FIGS. 1, 2 and 3 using the two-dimensional
frequency plane (f.sub.H, f.sub.V), respectively. Assuming that the
horizontal pixel pitch is P.sub.H and the vertical pixel pitch is
P.sub.V, each diagram represents an area of 0.ltoreq.f.sub.H
.ltoreq.1/P.sub.H and 0.ltoreq.f.sub.V .ltoreq.1/2P.sub.V. In any
diagram, the arrow indicates a carrier of each color, the arrow
length indicates a magnitude of the carrier, and the arrow
direction indicates a phase relationship.
In FIG. 4, color light carriers occur at (1/2P.sub.H, 0),
(1/P.sub.H, 0), (0, 1/4P.sub.V), (1/2P.sub.H, 1/4P.sub.V) and
(1/P.sub.H, 1/4P.sub.V) other than (0, 0). Among then, (0, 0) and
(1/P.sub.H, 0) represent the carriers which occur for achromatic
light and cause turn-back distortions. The remaining carriers are
perfectly canceled out and disappeared for achromatic light, but
not disappeared for chromatic light and cause turn-back
distortions.
Likewise, in FIG. 5, color light carriers occur at (1/2P.sub.H, 0),
(1/P.sub.H, 0), (1/2P.sub.H, 1/4P.sub.V), (0, 1/2P.sub.V),
(1/2P.sub.H, 1/2P.sub.V) and (1/P.sub.H, 1/2P.sub.V) other than (0,
0). Among then, (0, 0) and (1/P.sub.H, 0) represent the carriers
which occur for achromatic light and others represent the carriers
which are disappeared for chromatic light.
In FIG. 6, assuming that the horizontal offset amount of the
solid-state image sensor is P.sub.H /2, color light carriers occur
at (2/3P.sub.H, 0), (1/3P.sub.H, 1/2P.sub.V) and (1/P.sub.H,
1/2P.sub.V) Among them, (0, 0) and (1/P.sub.H, 1/2P.sub.V)
represent the carriers which occur for achromatic light and others
represent the carriers which are disappeared for chromatic
light.
It is generally known that the above offset sampling structure has
a feature as follows. With the rectangular sampling structure as
shown in FIGS. 1 and 2, since the carrier occurs for achromatic
light at the frequency corresponding to the horizontal position of
(1/P.sub.H, 0), f.sub.H =1/2P.sub.H becomes the Nyquist's frequency
and the frequency component thereabove cannot be obtained. Thus,
the horizontal resolution obtainable with this structure is
maximally f.sub.H =1/2P.sub.H. On the other hand, with the offset
sampling structure as shown in FIG. 3, the carrier does not occur
for achromatic light at the frequency corresponding to the
horizontal position of (1/P.sub.H, 0) and f.sub.H =1/P.sub.H can
become the Nyquist's frequency. Therefore, although the color
filter arrangement of FIG. 3 has the same sampling pitch as the
rectangular sampling structure of FIGS. 1 and 2, it can realize the
horizontal resolution twice the above cases, i.e., f.sub.H
=1/P.sub.H.
However, because objects to be usually photographed are not always
achromatic but colored in general cases, the color light carriers
at all the positions shown in FIGS. 4, 5 and 6 generate turn-back
distortions, so that some scenes may be awkward or hard to see.
This necessitates use of an optical low-pass filter or the like to
cut the detrimental color light carriers, which leads to a
reduction in the resolution.
In the color solid-state image sensing device with the offset
sampling structure shown in FIGS. 3 and 6, for example, the color
light carrier occurs at the position of (2/3P.sub.H, 0) and,
therefore, an optical low-pass filter capable of cutting off the
frequency component above f.sub.H =2/3P.sub.H in the horizontal
direction is required. This means that while the horizontal
resolution up to f.sub.H =1/P.sub.H could be intrinsically obtained
for achromatic light, the practically obtainable horizontal
resolution is only 2/3of that, i.e., f.sub.H =2/3P.sub.H.
SUMMARY OF THE INVENTION
The present invention has been made to solve the problem as
mentioned above, and its object is to provide a color image sensing
device which has good resolution and causes less moires, and which
can produce the horizontal resolution up to f.sub.H =1/P.sub.H even
with the sampling structure having the horizontal pixel pitch of
P.sub.H and the vertical pixel pitch of P.sub.V.
To achieve the above object, a color image sensing device according
to one embodiment of the present invention is constituted as
follows.
Specifically, the color image sensing device comprises an image
sensor having pixels of the offset sampling structure in which the
horizontal pitch is P.sub.H, the vertical pitch is P.sub.V and the
horizontal offset amount is P.sub.H /2, and a color filter array
comprising three or four types of color filters provided in
one-to-one relation to the pixels of the image sensor, the color
filter array having the offset sampling structure in which those
types of color filters are each arranged with the horizontal pitch
of 2P.sub.H, the vertical pitch of 2P.sub.V and the horizontal
offset amount of P.sub.H.
With the color image sensing device thus constituted, the
horizontal resolution up to f.sub.H =1/P.sub.H is obtained for
achromatic light and the horizontal and vertical resolutions are
less reduced for chromatic light.
Another embodiment of the present invention is to provide an image
sensing device in which the image sensor using a color filter array
of the offset sampling structure according to the above embodiment
is combined with an optical low-pass filter which is effective in
suppressing occurrence of color moires.
To achieve the above object, a color image sensing device according
to still another embodiment of the present invention is given by
(1) or (2) below.
(1) A color image sensing device featured in comprising the
following components a, b and c:
a. an image sensor having pixels of the offset sampling structure
in which the horizontal pitch is P.sub.H, the vertical pitch is
P.sub.V and the horizontal offset amount is P.sub.H /2;
b. a color filter array comprising four types of color filters
provided in one-to-one relation to the pixels of the above image
sensor, the color filter array having the offset sampling structure
in which those types of color filters are each arranged with the
horizontal pitch of 2P.sub.H, the vertical pitch of 2P.sub.V and
the horizontal offset amount of P.sub.H ; and
c. conversion means for converting outputs of the pixels associated
with the four types of color filters into RGB signals through
matrix operation of 3 row.times.4 column, the matrix being set such
that, for each row, the sum of coefficients of first two columns is
equal to the sum of coefficients of the remaining two columns.
(2) A color image sensing device according to (1) and including an
optical low-pass filter which is disposed in an image sensing
optical system and divides an incident light beam into twos spaced
through a distance D in a direction turned by an angle .theta.
counterclockwise relative to the scanning direction of the image
sensor or in an direction turned by an angle .theta. clockwise
relative to the reversed scanning direction of the image sensor,
the optical low-pass filter meeting the conditions below; ##EQU1##
where 0.ltoreq..theta..ltoreq..pi./2.
With the color image sensing devices of above (1) and (2), the
resolution up to f.sub.H =1/P.sub.H is obtained and moires are less
generated. To achieve the above object, a color image sensing
device according to still another embodiment of the present
invention is constituted as follows. Specifically, the color image
sensing device comprises the following components a, b and c:
a. an image sensor having the offset sampling structure in which
the horizontal pixel pitch is P.sub.H, the vertical pixel pitch is
P.sub.V and the horizontal pixel offset amount is P.sub.H /2;
b. a color filter array comprising three or more types of color
filters, provided for the above image sensor, and having the offset
sampling structure in which those types of color filters are each
arranged with the horizontal pitch of 2P.sub.H, the vertical pitch
of 2P.sub.V and the horizontal offset amount of P.sub.H ; and
c. an optical low-pass filter disposed in an image sensing optical
system and comprising a first optical member for dividing an
incident light beam into twos spaced through a distance P.sub.1 in
a direction turned by +45.degree. or -45.degree. counterclockwise
relative to the horizontal scanning direction of the image sensor
and a second optical member for dividing an incident light beam
into twos spaced through a distance P.sub.2 in a direction turned
by 90.degree. relative to the beam dividing direction of the first
optical member, the optical low-pass filter meeting the conditions
below; ##EQU2##
With the color image sensing device thus constituted, color
carriers generated by the color filter array are removed by the
optical low-pass filter and the occurrence of color moires is
suppressed.
To achieve the above object, a color image sensing device according
to still another embodiment of the present invention is constituted
as follows.
Specifically, the color image sensing device comprises the
following components a, b and c:
a. an image sensor having the offset sampling structure in which
the horizontal pixel pitch is P.sub.H, the vertical pixel pitch is
P.sub.V and the horizontal pixel offset amount is P.sub.H /2;
b. a color filter array comprising three or more types of color
filters, provided for the above image sensor, and having the offset
sampling structure in which those types of color filters are each
arranged with the horizontal pitch of 2P.sub.H, the vertical pitch
of 2P.sub.V and the horizontal offset amount of P.sub.H ; and
c. an optical low-pass filter disposed in an image sensing optical
system and comprising a first optical member for dividing an
incident light beam into twos spaced through a distance P.sub.1 in
a direction turned by +45.degree. or -45.degree. counterclockwise
relative to the horizontal scanning direction of the image sensor,
a second optical member for dividing an incident light beam into
twos spaced through a distance P.sub.2 in a direction parallel to
the horizontal scanning direction and a third optical member for
dividing an incident light beam into twos spaced through a distance
P.sub.3 in a direction turned by 90.degree. relative to the beam
dividing direction of the first optical member, the optical
low-pass filter meeting the conditions below; ##EQU3##
With the color image sensing device thus constituted, color
carriers generated by the color filter array are removed by the
optical low-pass filter and the occurrence of color moires is
suppressed.
Other objects and features of the present invention will be
apparent from the following detailed description and the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing one example of arrangement of color
filters;
FIG. 2 is a diagram showing another example of arrangement of color
filters;
FIG. 3 is a diagram showing still another example of arrangement of
color filters;
FIG. 4 is a characteristic diagram of color light carriers in the
color filter arrangement of FIG. 1;
FIG. 5 is a characteristic diagram of color light carriers in the
color filter arrangement of FIG. 2;
FIG. 6 is a characteristic diagram of color light carriers in the
color filter arrangement of FIG. 3;
FIG. 7 is a diagram showing an arrangement of color filters used in
a first embodiment of the present invention;
FIG. 8 is a characteristic diagram of color light carriers in the
color filter arrangement of FIG. 7;
FIG. 9 is a block diagram of the first embodiment;
FIG. 10 is a diagram for explaining characteristics of the first
embodiment;
FIG. 11 is a diagram showing an arrangement of color filters used
in a second embodiment of the present invention;
FIG. 12 is a characteristic diagram of color light carriers in the
color filter arrangement of FIG. 11;
FIG. 13 is a block diagram of the second embodiment;
FIG. 14 is a diagram showing layout of an optical low-pass filter
for use in a third embodiment of the present invention;
FIG. 15 is a diagram for explaining the construction of the optical
low-pass filter in the third embodiment;
FIG. 16 is a characteristic diagram of the optical low-pass filter
in the third embodiment;
FIG. 17 is a diagram showing a modification of the optical low-pass
filter in the third embodiment;
FIG. 18 is a characteristic diagram of the optical low-pass filter
shown in FIG. 17;
FIG. 19 is a diagram showing two-dimensional frequency
characteristics (in the second quadrant) of color light carriers in
the color filter arrangement of FIG. 7;
FIG. 20 is a diagram for explaining characteristics of a fourth
embodiment;
FIG. 21 is a diagram for explaining the construction of the optical
low-pass filter used in a fifth embodiment of the present
invention;
FIG. 22 is a characteristic diagram of the optical low-pass filter
in the fifth embodiment;
FIG. 23 is a characteristic diagram of a modification of the
optical low-pass filter in the fifth embodiment;
FIG. 24 is a diagram for explaining characteristics of a sixth
embodiment of the present invention;
FIGS. 25 and 26 are diagrams for explaining the offset sampling
structure;
FIG. 27 is a diagram showing two-dimensional frequency
characteristics of an ideal optical low-pass filter;
FIG. 28 is a diagram showing two-dimensional frequency
characteristics of an optical low-pass filter used in a seventh
embodiment of the present invention;
FIG. 29 is a diagram showing the construction of primary parts of
the seventh embodiment;
FIGS. 30A to 30C are diagrams for explaining operation of the
optical low-pass filter in the seventh embodiment;
FIG. 31 is a diagram showing the construction of primary parts of
an eighth embodiment of the present invention;
FIG. 32 is a diagram showing the construction of primary parts of a
ninth embodiment of the present invention;
FIG. 33 is a diagram showing the construction of primary parts of a
tenth embodiment of the present invention;
FIGS. 34A to 34C are diagrams for explaining operation of the
optical low-pass filter in the seventh embodiment;
FIG. 35 is a diagram showing the construction of primary parts of
an eleventh embodiment of the present invention;
FIG. 36 is a block diagram of a twelfth embodiment;
FIG. 37 is a block diagram of an adapted filter process for a
luminance signal in the twelfth embodiment;
FIGS. 38 and 39 are diagrams showing the construction of a filter
used in the twelfth embodiment;
FIGS. 40 and 41 are diagrams showing image patterns which are
liable to generate false colors;
FIG. 42 is a block diagram of a thirteenth embodiment;
FIG. 43 is a block diagram showing a matrix selection unit in the
thirteenth embodiment;
FIG. 44 is a table for explaining operation of the thirteenth
embodiment; and
FIG. 45 is a table for explaining operation of the thirteenth
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the present invention will be described in detail in
connection with preferred embodiments.
FIG. 7 shows an arrangement of color filters in a color image
sensing device according to a first embodiment of the present
invention. As will be seen from FIG. 7, a solid-state image sensor
has the offset sampling structure in which the horizontal pixel
pitch is P.sub.H, the vertical pixel pitch is P.sub.V and the
horizontal pixel offset amount is P.sub.H /2. The image sensor is
associated with a color filter array of the type that Magenta,
green, cyan and yellow light transmissive filters Mg, Gr, Cy, Ye
are arranged at positions corresponding to pixels in one-to-one
relation.
In the offset sampling structure, those color filters are each
arranged with the horizontal pitch of 2P.sub.H, the vertical pitch
of 2P.sub.V and the horizontal offset amount of P.sub.H.
FIG. 8 is a characteristic diagram representing color light
carriers in the above color filter arrangement using the
two-dimensional frequency plane (f.sub.H, f.sub.V) over an area of
0.ltoreq.f.sub.H .ltoreq.1P.sub.H and 0.ltoreq.f.sub.V 11/2P.sub.V
in the first quadrant. In FIG. 8, the color light carriers occur at
(1/P.sub.H, 0), (1/2P.sub.H, 1/4P.sub.V), (0, 1/2P.sub.V) and
(1/P.sub.H, 1/2P.sub.V) other than (0, 0). Among them, (0, 0) and
(1/P.sub.H, 0) represent the carriers which occur for achromatic
light and the remaining carriers are disappeared for achromatic
light.
As will be apparent from FIG. 8, no color light carriers occur
until the position of (1/P.sub.H, 0) in the horizontal direction.
Accordingly, the frequency component up to f.sub.H =1/P.sub.H can
be obtained. In other words, the color image sensing device of this
embodiment can produce the horizontal resolution as high as f.sub.H
=1/P.sub.H which is 1.5 times that obtainable with the color image
sensing device having the sampling structure of the prior art.
The color light carrier nearest to the origin is given by
(1/2P.sub.H, 1/4P.sub.V), but it is sufficiently spaced from the
origin and is also disappeared for achromatic light. Therefore,
that carrier will not cause a significant turn-back distortion for
both the horizontal and vertical frequency components.
Thus, with the color image sensing device of this embodiment, high
resolution is obtained and moires are less caused.
Signal processing in the color image sensing device which has the
color filter arrangement shown in FIG. 1 will be next described.
FIG. 9 is a block diagram for the signal processing.
A CCD sensor 1 as a solid-state image sensor is associated with a
color filter array 19 comprising four types of color filters shown
in FIG. 7. A video signal read out of the sensor 1 pixel by pixel
is subjected to gain adjustment by an AGC 2 and, thereafter, to A/D
conversion by an A/D (analog-to-digital) converter 3 at the timing
synchronized to the read-out clock. The A/D converter 3 preferably
has linear characteristics for the purpose of color processing to
be carried out later, and the A/D conversion is desirably performed
using 8 or more bits from the stand-point of errors in
quantization.
A resulting luminance signal is passed through an interpolation
filter 4 for two-dimensionally interpolate the offset sampling
structure and a high-pass filter (HPF) 5 for detection of the
higher-frequency luminance component thereof. The detected
component is added in an adder 6 with the lower-frequency luminance
component Y.sub.L obtained by the later-described technique and,
thereafter, a summation signal is subjected to D/A
(digital-to-analog) conversion by a D/A converter 7 and then
outputted therefrom.
On the other hand, the output of the A/D converter 3 is also
applied to four interpolation filters 8, 9, 10 and 11 which
respectively output color signals Mg, Cy, Ye and Gr at the
synchronized timing.
Note that by controlling the sequence of horizontal lines applied
to the interpolation filters 8, 9, 10 and 11 in accordance with the
clock, it is possible to obtain both an output signal based on
interlace scan and an output signal based on non-interlace
scan.
The synchronized color signals from the interpolation filters 8 to
11 are inputted to an RGB conversion unit 12 for conversion into
three signals R, G, B. This conversion is performed through matrix
operation below. ##EQU4## Here, the matrix A is a matrix of 3 row
and 4 column optimized to make spectroscopic characteristics
Mg(.lambda.), Gr(.lambda.), Cy(.lambda.), Ye(.lambda.) of the
sensor 1 for Mg, Gr, Cy, Ye approach ideal spectroscopic
characteristics Mg(.lambda.), Gr(.lambda.), Cy(.lambda.) for RGB
specified in the NTSC system. Let it be now assumed that:
If the gain is adjusted to be constant for a monochromatic object,
the base band components of Mg, Gr, Cy, Ye can be expressed below
by using some function .alpha.(f.sub.H, f.sub.V) on the frequency
space:
It is here supposed that the base band components of respective
colors are sufficiently limited in band by two-dimensional filters
such as the interpolation filters 8 to 11. In this case, the
carrier components of the color signals at the position
(1/2P.sub.H, 1/4P.sub.V) in FIG. 8 are expressed below:
Therefore, the carrier components of the RGB signals at that
position are expressed below from Equations (1), (2), (3) and (4).
##EQU5## At this time, if for each row of the matrix A the sum of
coefficients of the first and third columns is equal to the sum of
coefficients of the second and fourth columns, i.e., if the
relationship of:
holds, the carrier components of the RGB signals at the point
(1/2P.sub.H, 1/4P.sub.V) are reduced and, as result, there occur no
carrier components of the color signals at that point. Likewise,
the carrier components of the color signals at the point
(-1/2P.sub.H, -1/4P.sub.V) in the third quadrant, symmetrical to
the point (1/2P.sub.H, 1/4P.sub.V) about the origin in FIG. 8, can
also be disappeared by making the relationship of Equation (6)
satisfied.
FIG. 10 shows the positions at which the carriers of the luminance
signal and the color signals occur near the origin. Because of
disappearance of the carriers at (+1/2P.sub.H, 1/4P.sub.V) (double
signs being effective for the same order), turn-back distortions to
the base bands become smaller than the case of not meeting the
relationship of Equation (6).
As explained above, the RGB conversion unit shown in FIG. 9
converts the color signals Mg, Gr, Cy, Ye into the RGB signals
while reducing turn-back distortions.
Then, in a white balance unit 13, the RGB signals are converted
from R, G, B into .alpha.R, G, .beta.B based on color temperature
information obtained by a white balance sensor 18, thereby making
white balance.
After that, in a .gamma. conversion unit 14, the RGB signals are
subjected to .gamma. conversion through table conversion. In a
color difference matrix unit 113, the following conversion in
conformity with the NTSC system is performed for producing the
aforesaid lower-frequency luminance component Y.sub.L and color
difference signals R-Y, B-Y; ##EQU6## The color difference signals
R-Y, B-Y are subjected to D/A conversion by subsequent D/A
converters 16, 17, respectively, and then outputted therefrom. The
lower-frequency luminance component Y.sub.L is added with the
higher-frequency luminance component detected by the high-pass
filter 5 as mentioned above, following which the summation signal
is subjected to D/A conversion by a D/A converter 7 and then
outputted therefrom.
In this embodiment, the above signal processing system may be
hard-wired according to the block diagram, or it may be implemented
in the form of software using a DSP (digital signal processor) or
the like.
A second embodiment of the present invention will be next
described.
FIG. 11 shows an arrangement of color filters making up a color
filter array used in a color solid-state image sensing device
according to a second embodiment of the present invention. As with
the first embodiment, a solid-state image sensor of this embodiment
has the offset sampling structure in which the horizontal pixel
pitch is P.sub.H, the vertical pixel pitch is P.sub.V and the
horizontal pixel offset amount is P.sub.H /2. Color filters, i.e.,
red, green and blue light transmissive filters Rd, Gr, Bl are
arranged at positions corresponding to pixels in one-to-one
relation. In the color filter array, as shown in FIG. 10, the color
filters Rd and Bl are each arranged with the offset sampling
structure having the horizontal pitch of 2P.sub.H, the vertical
pitch of 2P.sub.V and the horizontal offset amount of P.sub.H. On
the other hand, while the color filter Gr is arranged in the form
of oblique stripes, this arrangement can be regarded as that two
arrays of the offset sampling structure having the horizontal pitch
of 2P.sub.H, the vertical pitch of 2P.sub.V and the horizontal
offset amount of P.sub.H are superposed with shifts of P.sub.H /2
in the horizontal direction and P.sub.V in the vertical direction
relative each other. FIG. 12 is a characteristic diagram
representing color light carriers in the above color filter
arrangement using the two-dimensional frequency plane (f.sub.H,
f.sub.V) over an area of 0<f.sub.H <1/P.sub.H and
0<f.sub.V <1/2P.sub.V in the first quadrant.
In FIG. 12, the color light carriers occur at (1/P.sub.H, 0),
(1/2P.sub.H, 1/4P.sub.V), (0, 1/2P.sub.V) and (1/P.sub.H,
1/2P.sub.V) other than (0, 0). Among them, (0, 0) and (1/P.sub.H,
0) represent the carriers which occur for achromatic light and the
remaining carriers are disappeared for achromatic light. This is
similar to the first embodiment.
Accordingly, also in this embodiment, the horizontal resolution as
high as f.sub.H =1/P.sub.H can be obtained. In addition, no
significant turn-back distortion occurs for both the horizontal and
vertical frequency components.
Signal processing in the color solid-state image sensing device
which has the color filter arrangement shown in FIG. 11 will be
next described by referring to FIG. 13.
A CCD sensor 201 is associated with a color filter array 201a
comprising three types of color filters shown in FIG. 11. A video
signal read out of the sensor 201 pixel by pixel is subjected to
gain adjustment by an AGC 202 and, thereafter, gains of RGB signals
are adjusted in a white balance unit 211 based on color temperature
information obtained by a white balance sensor 220, thereby making
white balance. After that, the RGB signals are subjected to .gamma.
conversion in a .gamma. conversion unit 212, and then to A/D
conversion by an A/D converter 203 at the timing synchronized to
the read-out clock.
A resulting luminance signal is passed through an interpolation
filter 225 for two-dimensionally interpolating the offset sampling
structure and then a high-pass filter (HPF) 216 for detection of
the higher-frequency luminance component thereof. The detected
component is added in an adder 217 with the lower-frequency
luminance component Y.sub.L obtained by the later-described
technique and, thereafter, a summation signal is subjected to D/A
conversion by a D/A converter 218 and then outputted therefrom.
On the other hand, the output of the A/D converter 203 is also
applied to three interpolation filters 206, 207 and 208 which
respectively output color signals R, G and B at the synchronized
timing.
Note that by controlling the sequence of horizontal lines applied
to the interpolation filters 206, 207, 208 and 225 in accordance
with the clock, it is possible to obtain both an output signal
based on interlace scan and an output signal based on non-interlace
scan.
The synchronized color signals from the interpolation filters 206,
207 and 208 are inputted to a color difference matrix unit 213 for
conversion in conformity with the NTSC according to the equation
(1), to thereby produce the lower-frequency luminance component
Y.sub.L and color difference signals R-Y, B-Y. The color difference
signals R-Y, B-Y are subjected to D/A conversion by subsequent D/A
converters 214, 215, respectively, and then outputted therefrom.
The lower-frequency luminance component Y.sub.L is added with the
higher-frequency luminance component detected by the high-pass
filter 216 as mentioned above, following which the summation signal
is subjected to D/A conversion by a D/A converter 218 and then
outputted therefrom.
In this embodiment, too, the above signal processing system may be
hard-wired according to the block diagram, or it may be implemented
in the form of software using a DSP or the like.
While the above embodiments have been explained in connection with
still picture recording, the present invention is not limited
thereto and can also be practiced in motion picture recording such
as represented by video cameras.
With this embodiment, as explained above, in spite of using the
sampling structure in which the horizontal pixel pitch is P.sub.H
and the vertical pixel pitch is P.sub.V, no detrimental carriers
occur over a wide range along the horizontal frequency axis, and
the horizontal resolution as high as the frequency of f.sub.H
=1/P.sub.H. It is thus possible to provide the color image sensing
device which has high resolution and causes fewer moires.
Meanwhile, as stated before, in the color image sensing device of
the first embodiment there occur carrier components responsible for
turn-back distortions at such positions as shown in FIG. 10. In
order to minimize the turn-back distortions in that type color
image sensing device, an optical low-pass filter 50 is required to
be located forwardly of the sensor 1, as shown in FIG. 14, to
thereby trap positions in the vicinity of the carriers
(.-+.1/2P.sub.H, .+-.1/4P.sub.V) (double signs being effective for
the same order), which are closest to the origin, for suppressing
the turn-back distortions.
A color image sensing device comprising the image sensing optical
system of the first embodiment and such as additional optical
low-pass filter will be described below as a third embodiment of
the present invention. The optical low-pass filter 50 functions to,
as shown in FIG. 15, divide an incident light beam into twos spaced
through a distance D in a direction turned by an angle .theta.
clockwise relative to the reversed scanning direction, while
meeting the conditions below: ##EQU7## where
If the value of D exceeds a lower limit of Inequality (7), the
turn-back distortions would be increased and, if it exceeds an
upper limit of Inequality (7), the resolution would be decreased.
Thus, any case would lead to the unsatisfactory result.
In this embodiment, .theta. and D are set as follows: ##EQU8## By
so setting, the optical low-pass filter 50 traps such positions as
indicated by dot lines in FIG. 16. Therefore, the carriers of the
color signals are located at not only (.-+.1/2P.sub.H,
.+-.1/4P.sub.V) (double signs being effective for the same order),
but also (.+-.1/P.sub.H, 0) and (0, .+-.1/2P.sub.V), making it
possible to minimize the turn-back distortions caused by the color
signals and thus provide the satisfactory image quality.
The exact same characteristics as the above can also be obtained in
the case of setting the beam dividing direction of the optical
low-pass filter 50 in a direction symmetrical to the direction
shown in FIG. 15 about a point, i.e., in a direction turned by an
angle .theta. clockwise relative to the scanning direction. The
optical low-pass filter 50 can be made of a double refracting plate
using a uniaxial crystal such as quartz. Other than this example,
the filter 50 may be in any form, including a prism, so long as it
has a property to divide the incident light beam into twos.
Furthermore, as shown in FIG. 17, there can also be employed, as a
modification of the third embodiment, an optical low-pass filter 80
in combination of an optical low-pass filter 81 which is made of a
double refracting plate and divides an incident light beam into
twos spaced through a distance D below in a direction turned by an
angle 45.degree. clockwise relative to the reversed scanning
direction; ##EQU9## and an optical low-pass filter 82 which is made
of a double refracting plate for dividing an incident light beam
into twos spaced through a distance P.sub.H /2 parallel to the
scanning direction. In this case, spatial frequency characteristics
of the optical low-pass filter 80 trap such positions as indicated
by dot lines in FIG. 18. Therefore, not only the carriers of the
color signals located at (.-+.1/2P.sub.H, .+-.1/4P.sub.V) (double
signs being effective for the same order) are trapped, but also the
carriers of the color signals located at (.+-.1/P.sub.H, 0) and (0,
.+-.1/2P.sub.V) are trapped in their vicinity, making it possible
to achieve a sufficient degree of suppression. In addition, since
the carriers of the luminance signal located at (.+-.1/P.sub.H,
.+-.1/2P.sub.V) (double signs being effective regardless of the
order) are further trapped, there can be obtained images in which
the turn-back distortions are satisfactorily suppressed.
A fourth embodiment of the present invention will be next
described. This embodiment is different from the first embodiment
only in the matrix for use with the RGB conversion unit.
FIG. 19 is a characteristic diagram representing color light
carriers in the color solid-state image sensor of FIG. 7 using the
two-dimensional frequency plane (f.sub.H, f.sub.V) in the second
quadrant. In FIG. 19, the color light carriers occur at
(-1/P.sub.H, 0), (-1/2P.sub.H, 1/4P.sub.V), (0, 1/2P.sub.V) and
(-1/P.sub.H, 1/2P.sub.V) other than (0, 0). Among them, (0, 0) and
(-1/P.sub.H, 1/2P.sub.V) represent the carriers which occur for
achromatic light and the remaining carriers which occur for
chromatic light. The above carrier positions are symmetrical to
those shown in FIG. 8 about the f.sub.V axis except for carrier
signs of the respective colors only at (-1/2P.sub.H,
1/4P.sub.V).
When converting the color signals Mg, Gr, Cy, Ye into the RGB
signals in the RGB conversion unit 110 shown in FIG. 9, the carrier
components of the color signals at the position (-1/2P.sub.H,
1/4P.sub.V) in FIG. 19 are expressed below using Equation (3):
Therefore, the carrier components of the RGB signals at that
position are expressed below from Equations (1), (2), (3) and (10):
##EQU10## At this time, if for each row of the matrix A the sum of
coefficients of the first and fourth columns is equal to the sum of
coefficients of the second and third columns, i.e., if the
relationship of:
(i=1, 2, 3) (12)
holds, the carrier components of the RGB signals at the point
(-1/2P.sub.H, 1/4P.sub.V) are disappeared and, as a result, there
occur no carrier components of the color signals at that point.
Likewise, the carrier components of the color signals at the point
(1/2P.sub.H, -1/4P.sub.V) in the fourth quadrant, symmetrical to
the point (-1/2P.sub.H, 1/4P.sub.V) about the origin in FIG. 19,
can also disappear by satisfying the relationship of Equation
(12).
FIG. 20 shows the positions at which the carriers of the luminance
signal and the color signals occur near the origin.
In order to minimize the turn-back distortions in that type of
color image sensing device, like the first embodiment, the optical
low-pass filter 50 is required to be located forwardly of the
sensor 1, as shown in FIG. 14, to thereby trap positions in the
vicinity of the carriers (.+-.1/2P.sub.H, .+-.1/4P.sub.V) (double
sings being effective for the same order), which are closest to the
origin, for suppressing the turn-back distortions.
A color image sensing device comprising the image sensing optical
system of the fourth embodiment and such as additional optical
low-pass filter will be described below as a fifth embodiment of
the present invention. The optical low-pass filter 50 functions to,
as shown in FIG. 21, divide an incident light beam into twos spaced
through a distance D in a direction turned by an angle .theta.
counter-clockwise relative to the scanning direction, while meeting
the conditions below: ##EQU11## where
If the value of D exceeds a lower limit of Inequality (7), the
turn-back distortions would be increased and, if it exceeds an
upper limit of Inequality (7), the resolution would be decreased.
Thus, exceeding the limits would lead to the unsatisfactory result
in any case.
In this embodiment, .theta. and D are set as follows: ##EQU12## By
so setting, the optical low-pass filter 50 traps such positions as
indicated by dot lines in FIG. 22. Therefore, the carriers of the
color signals located at not only (.+-.1/2P.sub.H, .+-.1/4P.sub.V)
(double signs being effective for the same order), but also
(.+-.1/P.sub.H, 0) and (0, .+-.1/2P.sub.V), making it possible to
minimize the turn-back distortions caused by the color signals and
thus provide the satisfactory image quality.
The exactly same characteristics as the above can also be obtained
in the case the setting the beam dividing direction of the optical
low-pass filter 50 in a direction symmetrical to the direction
shown in FIG. 21 about a point, i.e., in a direction turned by an
angle .theta. counter-clockwise relative to the reversed scanning
direction. The optical low-pass filter 50 can be made of a double
refracting plate using an uniaxial crystal such as quartz. Other
than this example, the filter 50 may be in any form, including a
prism, so long as it has a property to divide the incident light
beam into twos. In order to provide the same effect as the optical
low-pass filter shown in FIG. 17, it is only required to use the
optical low-pass filter 80 in combination with the optical low-pass
filter 81 modified such that it is made of a double refracting
plate for dividing an incident light beam into twos spaced through
a distance D below in a direction turned by an angle 45.degree.
counter-clockwise relative to the scanning direction: ##EQU13## and
the optical low-pass filter 82 which is made of a double refracting
plate for dividing an incident light beam into twos spaced through
a distance P.sub.H /2 parallel to the scanning direction. In this
case, spatial frequency characteristics of the optical low-pass
filter 80 trap such positions as indicated by dot lines in FIG. 23.
Therefore, not only the carriers of the color signals located at
(.+-.1/2P.sub.H, .+-.1/4P.sub.V) (double signs being effective for
the same order) are trapped, but also the carriers of the color
signals located at (.+-.1/P.sub.H, 0) and (0, .+-.1/2P.sub.V) are
trapped in their vicinity making it possible to achieve a
sufficient degree of suppression. In addition, since the carriers
of the luminance signal located at (.+-.1/P.sub.H, .+-.1/2P.sub.V)
(double signs being effective regardless of the order) are further
trapped, there can be obtained images in which the turn-back
distortions are satisfactorily suppressed.
A sixth embodiment of the present invention will be next
described.
The carrier components of the color signals at the positions
(1/P.sub.H, 0) and (0, 1/2P.sub.V) in FIG. 8 are expressed below
using Equation (3):
Therefore, after conversion into the RGB signals by the RGB
conversion unit 12 shown in FIG. 9, the carrier components of the
RGB signals at those positions are expressed below from Equations
(1), (2), (3) and (13): ##EQU14## At this time, if for each row of
the matrix A the sum of coefficients of the first and second
columns is equal to the sum of coefficients of the third and fourth
columns, i.e., if the relationship of;
holds, the carrier components of the RGB signals at the points
(1/P.sub.H, 0) and (0, 1/2P.sub.V) disappear and, as a result,
there occur no carrier components of the color signals at that
point. Likewise, the carrier components of the color signals at the
points (-1/P.sub.H, 0) and (0, -1/2P.sub.V), symmetrical to the
point (1/2P.sub.H, 0) and (0, 1/2P.sub.V) about the origin in FIG.
8, can also disappear by satisfying the relationship of Equation
(15) satisfied.
FIG. 24 shows the positions at which the carriers of the luminance
signal and the color signals occur near the origin.
In order to minimize the turn-back distortions in that type color
image sensing device, it is only required to combine the optical
low-pass filter used in the third embodiment which has been
dividing characteristics as shown in FIG. 15, with the optical
low-pass filter used in the fifth embodiment which has beam
dividing characteristics as shown in FIG. 21. In this case, the
dividing distance in each of the optical low-pass filters is to
meet the conditions given by Equation (7).
With this embodiment, as explained above, the horizontal resolution
as high as the frequency of f.sub.H .ltoreq.1/P.sub.H, making it
possible to provide the color image sensing device which has high
resolution and causes fewer moires.
A seventh embodiment of the present invention will be next
described.
This embodiment is intended to provide a color image sensing device
in which the image sensor using the color filter arrangement of
such offset sampling structure as shown in FIG. 7 or 11 is combined
with an optical low-pass filter effective in suppressing color
moires.
In other words, this embodiment has been made by carefully
analyzing the sampling structure used in the color filter
arrangement shown in FIG. 7 or 11 from the two-dimensional aspect
and, as a result, deriving conditions of the optical low-pass
filter optimum for preventing color moires.
First, the offset sampling structure is considered in which the
horizontal and vertical pitches are P.sub.H and P.sub.V,
respectively, and the horizontal offset amount is P.sub.H /2, as
shown in FIG. 25. It is known that the above sampling structure
corresponds to, in terms of the two-dimensional frequency plane,
the offset structure in which the horizontal and vertical pitches
are 2/P.sub.H and 1/P.sub.V, respectively, and the horizontal
offset amount is 1/P.sub.H 2 as shown in FIG. 26. How to derive the
latter from the former is described in, for example, a book by
Takahiko Suisaka, "Digital Signal Processing of Image", Nikkan
Kogyo Shinbunsha (1985), p. 317.
Accordingly, assuming that the luminance signal is obtained based
on the so-called switch-Y method, by which the color signal from
the color filter for each pixel is regarded equivalently as the
luminance signal, in the image sensor using the color filter
arrangement as shown in FIG. 7 or 11, the sampling structure of the
luminance signal is represented by marks in FIG. 27. With regards
to the color signals, it is found that Mg in FIG. 7, for example,
also has the offset sampling structure in which the horizontal and
vertical pitches are 2P.sub.H and 2P.sub.V, respectively, and the
horizontal offset amount is P.sub.H. Therefore, the sampling
structure of the color signals is represented by mark .quadrature.
in FIG. 27.
As a result, an optical low-pass filter optimum for preventing
color moires should be such that its frequency characteristics
become zero or minimum near four sides of a rhombus indicated by
solid lines in FIG. 27.
Thus, in view of the foregoing, this embodiment is directed to a
color image sensing device which includes an optical filter optimum
for the image sensor using the color filter arrangement as shown in
FIG. 7 or 11.
The seventh embodiment will be described below in more detail. In
the following description, the term "directions" indicates two
opposite directions different from each other by 180.degree. and
the term "direction" indicates one direction.
While ideal characteristics are rhombic as mentioned above, when
constituting the optical low-pass filter by using a double
refracting plate or the like in practice, it is easier to make
frequency characteristics zero along four straight lines.sub.1,
l.sub.1 ', l.sub.2, l.sub.2 ' passing the color carrier frequencies
(1/2P.sub.H, 1/4PV), which are closest to the origin, the making
angles 45.degree. with respect to the fx axis, as indicated by
solid lines in FIG. 28. For this purpose, the optical low-pass
filter is required to comprise a first optical member for dividing
an incident light beam into twos spaced through a distance P.sub.1
in directions turned by an angle+45.degree. or -45.degree.
counter-clockwise relative to the horizontal scanning direction,
and a second optical member for dividing an incident light beam
into twos spaced through a distance P.sub.2 in directions making
90.degree. relative to the beam dividing directions of the first
optical member. In this case, the two-dimensional MTF value
representing frequency characteristics of the optical low-pass
filter is given by: ##EQU15##
Thus, the MTF value changes in the form of a cosine function in
directions making .+-.45.degree. relative to the horizontal and in
the form of a square of cosine as a result of multiplication of two
terms in the horizontal and vertical directions. Consequently, the
turn-back distortions are satisfactorily suppressed in the
horizontal, vertical and oblique directions.
To make the optical low-pass filter exhibit sufficient performance,
it is desirable that the four straight lines shown in FIG. 28 pass
near the points (.+-.1/2P.sub.H, .+-.1/4P.sub.V). For this purpose,
since the distances from the origin to the four straight lines are
given by;
values of these distances are required to fall within a range of
.+-.25% about h.sub.0 below: ##EQU16## Here, h.sub.0 represents the
distance from the origin to the four straight lines l.sub.1,
l.sub.1 ', l.sub.2, l.sub.2 ' when these lines pass the points
(.+-.1/2P.sub.H, .+-.1/4P.sub.V). To this end, the separation (or
division) widths P.sub.1, P.sub.2 are required to meet the
following conditions: ##EQU17##
In each equation, if the value exceeds an upper limit, the quantity
of color moires would be increased and, if it exceeds a lower
limit, the resolution of the entire system comprising the optical
low-pass filter and the image sensor would be decreased. Thus,
exceeding the limits would lead to the unsatisfactory result in any
case.
FIG. 29 shows the construction of primary parts of the color image
sensing device implementing the seventh embodiment of the present
invention. In FIG. 29, denotes by 21 is an optical low-pass filter
and 22 is a color image sensor having the offset sampling
structure, the image sensor being associated with a color filter
array 22a which has the color filter arrangement as shown in FIG. 7
or 11.
The optical low-pass filter 21 comprises a first optical member 23
consisted of double refracting plates 25, 26 and a second optical
member 24 made of a double refracting plate 27. As shown, the
separating directions of the double refracting plates 25, 26, 27
are turned by 90.degree., 0.degree., 45.degree. counter-clockwise
relative to the horizontal scanning direction and the magnitudes of
the separation widths are ##EQU18## respectively. FIGS. 30A to 30C
show how the light beam is separated successively by the optical
low-pass filter 21 of FIG. 29. As shown in FIG. 30A, a light beam
incident upon the double refracting plate 25 emerges therefrom as
two linearly polarized beams, i.e., an ordinary ray 38 and an
extraordinary ray 39, which have the equal intensity and are
polarized in the directions indicated by solid lines in the
drawing. Then, those two rays enter the double refracting plate 26
having the optical axis inclined by 90.degree. relative to the
optical axis of the double refracting plate 25. At this time, since
the light beam 39 becomes an ordinary ray for the double refracting
plate 26, it goes straight forward through the plate 26 and emerges
therefrom as is. On the other hand, since the light beam 38 becomes
an extraordinary ray for the double refracting plate 26, it is
refracted and emerges therefrom at a position denoted by 30. The
above process is illustrated in FIG. 3B. From the first optical
member 23 comprising the double refracting plates 25, 26, there are
eventually obtained two light beams 39, 30 separated in directions
turned by -45.degree. counter-clockwise relative to the horizontal
scanning direction with the separation width of P.sub.1, as shown.
Since those light beams 39, 30 are respectively polarized by
90.degree. and 0.degree. counter-clockwise relative to the
horizontal scanning direction, they are separated by the double
refracting plate 27 into fours which have the equal intensity, as
illustrated in FIG. 30C. The separation width effected by the
double refracting plate 27 is P.sub.2. The values of P.sub.1 and
P.sub.2 are set to meet above Equations (2) and (3), respectively,
for achieving the desired performance.
FIG. 31 shows the construction of primary parts of an eighth
embodiment of the present invention. A color image sensor 42 and a
color filter array 42a associated therewith are constituted in the
same manner as the seventh embodiment. An optical low-pass filter
90 comprises a first optical member 92 consisted of a phase plate
94 for converting linearly polarized light into circularly
polarized light and a double refracting plate 95, and a second
optical member 91 made of a double refracting plate 93. The first
optical member 92 and the second optical member 91 are not
necessarily arranged in the above order from the side of the image
sensor 42, and may be arranged in the reversed order. Two linearly
polarized beams, i.e., an ordinary ray and an extraordinary ray,
separated by the double refracting plate 93 with the separation
width of P.sub.2 are converted by the phase plate 94 into two
circularly polarized beams which are then separated by the double
refracting plate 95, having the separation width of P.sub.1, into
fours of the equal intensity similarly to the case of in FIG.
30C.
FIG. 32 shows the construction of primary parts of a ninth
embodiment of the present invention. An optical low-pass filter 100
in this embodiment uses a double refracting plate 96 which has the
separation width sufficiently smaller than the two double
refracting plates 93, 95 and also has the separating directions
turned by 0.degree. counterclockwise relative to the horizontal
scanning direction, in place of the phase plate 94 in the optical
low-pass filter 90 of FIG. 31. Assuming that the separation width
of the double refracting plate 96 is d.sub.3, there emerge from the
optical low-pass filter 100a total of eight light beams resulted
from double-shifting of the four light beams in FIG. 30C by d.sub.3
in the directions of 45.degree.. Because of d.sub.3 being
sufficiently small, however, the resulting frequency
characteristics are almost the same as those obtained in FIG. 30C.
With the ninth embodiment, dependency of the frequency
characteristics on wavelength is reduced as compared with the
eighth embodiment using the phase plate 94 and the thickness of the
entire device is reduced for more compact size.
In the foregoing embodiments, the optical member is constituted by
a single double refracting plate, a plurality of double refracting
plates, or a combination of a phase plate therewith. However, the
optical member for use in the present invention is not limited to
those embodiments and may be in any form, e.g., a prism placed in
the optical system, so long as it has a capability to divide the
light beam into twos.
With the seventh to ninth embodiments, as explained above, there
can be obtained an optical low-pass filter which is combined with a
color single-plate image sensor having the offset sampling
structure to effectively prevent the occurrence of color
moires.
A tenth embodiment of the present invention will be next described.
This tenth embodiment is directed to a color image sensing device
which includes an optimum optical filter as with the above seventh,
eighth and ninth embodiments.
The tenth embodiment will be explained below in more detail. In the
following description, the term "directions" indicates two opposite
directions different from each other by 180.degree. and the term
"direction" indicates one direction.
While ideal characteristics are rhombic as mentioned above, when
constituting the optical low-pass filter by using a double
refracting plate or the like in practice, it is easy to make
frequency characteristics zero along not only four straight lines
l.sub.1, l.sub.1 ', l.sub.3, l.sub.3 ' passing the color carrier
frequencies (.+-.1/2P.sub.H, .+-.1/4P.sub.V), which are closest to
the origin, and making angles .+-.45.degree. with respect to the
f.sub.x axis, but also straight lines l.sub.2, l.sub.2 ' parallel
to the f.sub.y axis, as indicated by solid lines in FIG. 28. For
this purpose, the optical low-pass filter is required to comprise a
first optical member for dividing an incident light beam into twos
spaced through a distance P.sub.1 in directions turned by an angle
.+-.45.degree. or -45.degree. counterclockwise relative to the
horizontal scanning direction, a second optical member for dividing
an incident light beam into twos spaced through a distance P.sub.2
in directions parallel to the horizontal scanning direction, and a
third optical member for dividing an incident light beam into twos
spaced through a distance P.sub.3 in directions making 90.degree.
relative to the beam dividing directions of the first optical
member. In this case, the two-dimensional MTF value representing
frequency characteristics of the optical low-pass filter is given
by: ##EQU19##
Accordingly, the MTF value can trap the vicinity of
(.+-.1/2P.sub.H, .+-.1/4P.sub.V) where the color signal carriers
occur, without deteriorating the horizontal resolution, and also
trap the vicinity of (.+-.1/P.sub.H, .+-.1/2P.sub.V) where the
luminance signal carriers occur. It is thus possible to suppress
all the carriers of the color and luminance signals which are
nearest to the origin.
To make the optical low-pass filter exhibit sufficient performance,
since the distance from the origin to the four straight lines shown
in FIG. 28 are given by;
values of these distances are required to fall within a range of
+25% about h.sub.0 below; ##EQU20## and the distance from the
origin to the two straight lines;
is required to fall within a range of .+-.25% about 1/P.sub.H.
Here, the value of h.sub.0 represents the distance from the origin
to the four straight lines l.sub.1, l.sub.1 ', l.sub.2, l.sub.2 '
when these lines pass the points (.+-.1/2P.sub.H,
.+-.1/4P.sub.V).
For that purpose, the separation (or division) widths P.sub.1,
P.sub.2, P.sub.3 are required to meet the following conditions:
##EQU21##
In each equation, if the value exceeds an upper limit, the quantity
of color moires caused by the carriers of the luminance and color
signals would be increased and, if it exceeds a lower limit, the
resolution of the entire system comprising the optical low-pass
filter and the image sensor would be decreased. Thus, exceeding the
limits would lead to the unsatisfactory result in any case.
FIG. 33 shows the construction of primary parts of the color image
sensing device implementing the tenth embodiment of the present
invention. In FIG. 33, denoted by 51 is an optical low-pass filter
and 52 is a color solid-state image sensor having the offset
sampling structure, the image sensor being associated with a color
filter array 52a which has the color filter arrangement as shown in
FIG. 7 or FIG. 11.
The optical low-pass filter 51 comprises first, second and third
optical members 53, 54, 55 made of double refracting plates 56, 57,
58, respectively. As shown, the separating directions of the double
refracting plates 56, 57, 58 are turned by 45.degree., 0.degree.,
-45.degree. counterclockwise relative to the horizontal scanning
direction and the magnitudes of the separation widths are ##EQU22##
respectively.
FIGS. 34A to 34C show how the light beam is separated successively
by the optical low-pass filter 1 of FIG. 35. As shown in FIG. 34A,
an incident light beam is divided by the double refracting plate 56
in directions turned by 45.degree. counterclockwise relative to the
horizontal scanning direction to emerge therefrom as two linearly
polarized beams, i.e., an ordinary ray 69 and an extraordinary ray
60, which have the equal intensity and are polarized in the
directions indicated by solid lines in the drawing. Then, those two
rays 69, 60 emerging from the double refracting plate 56 are
divided by the double refracting plate 57 in parallel to the
horizontal scanning direction into fours 69, 60, 61, 62, as shown
in FIG. 34B. The polarizing directions are indicated by solid
arrows in the drawing. Subsequently, as shown in FIG. 34C, the
light beams 69, 60, 61, 62 emerging from the double refracting
plate 57 are divided in directions turned by -45.degree.
counterclockwise relative to the horizontal scanning direction into
eights 69, 60, 61, 62, 63, 64, 65, 66. Eventually, those eight
light beams emerge from the optical low-pass filter 51. In this
case, the two-dimensional MTF value representing frequency
characteristics of the optical low-pass filter is given by:
##EQU23## Accordingly, it becomes possible to satisfactorily
suppress the occurrence of moires caused by the carriers of the
luminance and color signals as seen from FIG. 28.
FIG. 35 shows the construction of primary parts of a color image
sensing device according to an eleventh embodiment of the present
invention. A color image sensor 120 and a color filter array 120a
associated therewith are constituted in the same manner as the
tenth embodiment. An optical low-pass filter 110 comprises a first
optical member 111 consisted of a double refracting plate 114 and a
phase plate 115 for converting linearly polarized light into
circularly polarized light, a second optical member 112 consisted
of a double refracting plate 116 and a phase plate 117, and a third
optical member 113 made of a double refracting plate 118. The
double refracting plate 114 divides an incident light beam into
twos, i.e., an ordinary ray and an extraordinary ray, spaced
through a distance P.sub.1 in directions turned by 45.degree.
counterclockwise relative to the horizontal scanning direction.
Those two rays emerging from the double refracting plate 114 are
converted by the phase plate 115 from linearly polarized light into
circularly polarized light. The light beams emerging from the phase
plate 115 are divided by the double refracting plate 116 through a
distance P.sub.2 in directions parallel to the horizontal scanning
direction, and then converted by the phase plate 117 from linearly
polarized light into circularly polarized light. The light beams
emerging from the phase plate 117 are divided by the double
refracting plate 118 through a distance P.sub.3 in directions
turned by -45.degree. counterclockwise relative to the horizontal
scanning direction. Eventually, a total of eight light beams emerge
from the optical low-pass filter 90 similarly to the case shown in
FIG. 34C. Therefore, the two-dimensional MTF value of the optical
low-pass filter 90 is expressed by above Equation
(1) and thus can provide the same frequency characteristics as the
tenth embodiment.
In the tenth and eleventh embodiments, the first, second and third
optical members are not necessarily arranged in the above order
from the object side, and may be arranged in any order so long as
the optical low-pass filter has frequency characteristics meeting
Equation (1). Furthermore, in those embodiments, the optical member
is constituted by a single double refracting plate or a combination
of a phase plate therewith. However, the optical member for use in
the present invention is not limited to those embodiments and may
be in any form, e.g., a prism placed in the optical system, so long
as it has a capability to divide the light beam into twos.
With the tenth and eleventh embodiments, as explained above, there
can be obtained an optical low-pass filter which is combined with a
color single-plate image sensor having the offset sampling
structure to effectively prevent the occurrence of color
moires.
Meanwhile, in the case of the aforementioned color image sensing
device of the first embodiment as shown in FIG. 9, i.e., when the
signal from the sensor 1 is simply subjected to low-pass filtering
by the interpolation filter 4 to produce the luminance signal,
there occurs the following phenomenon.
Generally, responses of four color filters Mg, Cy, Ye, Gr to some
constant object are a little different from one another. Therefore,
even if luminance of the object is constant, the arrangement
structure of FIG. 7 is reflected in the luminance signal resulted
from the signal processing through the system of FIG. 9 and thus
appears in the form of checker patterns.
When the four color filters are arranged as shown in FIG. 7,
characteristics of the color light carriers are given as shown in
FIG. 8. The above phenomenon can be regarded as a phenomenon caused
by the DC component turning back to the carrier position indicated
by P in FIG. 8. Preventing that phenomenon in the simplest manner
requires to carry out signal processing such that response of the
carrier at the point P in FIG. 8 becomes zero. For example, by
setting a luminance signal Y at the position of Gr.sup.(1) in FIG.
7 as follows; ##EQU24## a horizontal low-pass filter for trapping
the horizontal frequency component 1/2P.sub.H in FIG. 8 is
realized. Accordingly, the point P is also trapped and the checker
patterns are removed.
Also, by adding information data on every two pixels spaced from
each other by two pixels in the vertical direction, a vertical
low-pass filter for trapping the vertical frequency component
1/4P.sub.V can be realized to remove the checker patterns.
However, if the above horizontal or vertical processing is
performed, the horizontal or vertical band would be so limited as
to deteriorate the resolution.
A twelfth embodiment of the present invention is intended to solve
the foregoing problem and is directed to an image signal processor
which can remove the checker patterns in the image sensing device
with the filter as shown in FIG. 7, without reducing the
resolution.
To this end, the image signal processor of this embodiment is
constituted as follows:
(1) an image signal processor comprising decision means for
determining whether or not a constant region exhibiting small
luminance change in a predetermined direction is present within an
image, processing means for carrying out low-pass filtering of a
luminance signal in the predetermined direction, and selection
means for selecting the processing means to carry out the low-pass
filtering when the decision means determines the presence of the
constant region; or
(2) an image signal processor in which the decision means makes a
decision based on absolute values of outputs of horizontal and
vertical band-pass filters for the luminance signal.
With the image signal processors of (1) and (2), when the constant
region exhibiting small luminance changes in a predetermined
direction is present within an image, low-pass filtering of the
luminance signal is carried out in the predetermined direction.
With the image signal processors of (2), the division of the
constant region exhibiting small luminance changes is made based on
absolute values of outputs of horizontal and vertical band-pass
filters for the luminance signal.
The twelfth embodiment will be described below in more detail. FIG.
36 is a block diagram of a signal processor for a video camera
which implements the twelfth embodiment.
A sensor 301 is associated with the color filter array shown in
FIG. 7 for reading two rows of the array in the zigzag form during
one horizontal scan period (1H) as seen from the drawing.
Accordingly, all the pixels are read out once for one field.
Assuming now that the read-out clock has frequency of f.sub.s, an
output of the sensor 301 is changed in the order of Mg, Cy, Gr, Ye
for each clock. The output signal is applied to an analog
processing unit 302 for signal processing such as AGC (automatic
gain control), and then subjected to A/D (analog-to-digital)
conversion by an A/D converter 303. The A/D converter used
desirably has 10 or more bit precision.
The resulting digital signal is inputted to a later-described
adaptive filter processing unit 305 for a luminance signal to
create a high-frequency luminance signal free of checker patterns
as mentioned above. The resulting signal is applied to a luminance
processing unit 306 for standard video signal processing such as
.gamma. conversion and blanking, and then subjected to D/A
(digital-to-analog) conversion by a D/A converter 307. The
resulting analog signal is added in a synch signal adding unit 308
with a synch signal for a standard TV signal.
A switch 304 is operated to change over an input signal for each
half of the clock f.sub.s so that a signal Mg-Gr is outputted to S1
and a signal Cy-Ye is outputted to S2.
Those signals changed over to S1 and S2 are inputted to the
switches 309 and 310, respectively, so that they are separated into
signals Mg, Gr, Cy, Ye for respective signals. Since these color
signals each have the offset structure as shown in FIG. 7, lacked
information is interpolated by two-dimensional interpolation
filters 311, 312, 313, 314.
In a matrix operation unit 315, primary color signals R, G, B are
obtained from the four interpolated color signals Mg, Gr, Cy, Ye
through matrix operation below: ##EQU25## Here, the matrix A is a
matrix of 3 row and 4 column optimized to make spectroscopic
characteristics Mg(.lambda.), Gr(.lambda.), Cy(.lambda.),
Ye(.lambda.) of the sensor 201 for Mg, Gr, Cy, Ye approach ideal
spectroscopic characteristics Mg(.lambda.), Gr(.lambda.),
Cy(.lambda.) for RGB specified in the NTSC system.
The primary color signals R, G, B thus obtained are applied to a
color processing unit 316 for signal processing such as white
balance (WB) and .gamma. conversion, following which they are
inputted to a color difference matrix operation unit 317. The color
difference matrix operation unit 317 creates two color difference
signals R-Y, B-Y which are subjected to D/A conversion by D/A
converters 318, 319, respectively.
After that, the color difference signals are subjected to
orthogonal modulation by an encoder 320 for mixing in a mixing unit
321 with the signal, comprising the luminance signal Y and the
synch signal S, and the mixed result is outputted as the standard
TV signal.
FIG. 37 shows the construction of the adaptive filter processing
unit 305 for the luminance signal. The inputted luminance signal Y
is the result of reading the pixels in the zigzag form in the order
of Mg, Cy, Gr and Ye. Since carriers are present at positions
corresponding to the frequency (1/2P.sub.H) equal to 1/4 of the
clock f.sub.s in the horizontal direction and 240 TV lines
(1/4P.sub.V) in the vertical direction, as mentioned before, it is
required to determine the magnitudes of frequency components at
those carrier positions.
A horizontal band-pass filter 401 comprises, as shown in the lower
half of FIG. 38, a digital filter with five taps of (-1/2, 0, 1, 0,
-1/2). A vertical band-pass filter 402 comprises, as shown in the
lower half of FIG. 39, a digital filter with three taps of (-1/2,
1, -1/2) by using a 1H (horizontal scan period) memory.
A decision unit 403 outputs a select signal S to a switch 407
depending on absolute values of outputs of the horizontal band-pass
filter 401 and the vertical band-pass filter 402.
In response to the select signal S from the decision unit 403, the
switch 407 is able to select one of a through signal output 404, an
output of a horizontal low-pass filter 405, and an output of a
vertical low-pass filter 405. The horizontal low-pass filter 405
can be constituted by, as shown in the upper half of FIG. 38,
sharing delays and taps with the horizontal band-pass filter 401.
Here, the horizontal low-pass filter 405 comprises a low-pass
filter with five taps of (1/2, 0, 1, 0, 1/2) to make response zero
along the line T.sub.H in FIG. 8, thereby enabling to remove the
checker patterns. Also, the vertical low-pass filter 406 can be
constituted by, as shown in the upper half of FIG. 39, sharing the
1H memory and taps with the vertical band-pass filter 401. Here,
the vertical low-pass filter 406 comprises a low-pass filter with
three taps of (1/2, 1, 1/2) to make response zero along the line
T.sub.V in FIG. 8, thereby enabling to remove the checker patterns.
Operation of the decision unit 403 can be controlled in accordance
with the following table:
______________________________________ Case Selection
______________________________________ 1 A .andgate. B S1 2 .sup.--
A .andgate. B S2 3 A .andgate. .sup.-- B S3 4 .sup.-- A .andgate.
.sup.-- B S2 or S3 ______________________________________
It is here assumed that A represents an event that an absolute
value of the output of the horizontal band-pass filter 401 is
larger than a certain value, and B represents an event that an
absolute value of the output of the vertical band-pass filter 402
is larger than a certain value. A, B respectively represent
reversed events of A, B.
For example, in Case 1 where the object structure is largely
changed in both the horizontal and vertical directions, there occur
A and B. In this case, the checker patterns will not be awkward
even if they are present. Conversely, if the signals are subjected
to the low-pass filtering, the image would be blurred. Therefore,
the switch S11 is selected.
As another example, in Case 2 where the object has the vertical
component, the switch S12 is selected to effect the low-pass
filtering in the horizontal direction. By so doing, the checker
patterns can be removed without impairing information on the
object.
In the above embodiment, the outputs of the horizontal and vertical
band-pass filters are combined with each other to determine whether
or not the object structure is abruptly changed in the portion of
interest. Alternatively, it is also possible to determine changes
in the object structure in oblique directions by using oblique
band-pass filters and effect the oblique low-pass filtering if the
changes are small.
With this embodiment, as described above, the checker patterns can
be prevented without deteriorating the resolution.
Meanwhile, in the first embodiment, the color signals are processed
after conversion of the four complementary color signals Mg, Cy,
Ye, Gr, having been interpolated, into the primary color signals R,
G, B through the matrix operation, as shown in FIG. 9. This color
processing technique is called a matrix method or primary color
separating method and described in the specification of Japanese
Patent Application No. 63-281456 filed by the present applicant and
the article "Simultaneous RGB Processing in Image Mixing CCD
Camera" by Nishimura, etc., Technical Report of Television Society
of Japan, TEBS 89-9 ED 89-13, Feb. 1989.
In the RGB conversion unit 12 of FIG. 9, the four complementary
color signals Mg, Cy, Ye, Gr are converted into the primary color
signals R, G, B through the matrix operation below: ##EQU26##
On this occasion, the oblique pattern of bright and dark areas are
considered as shown in FIG. 40. The fat lines represent bright
areas and the thin lines represent dark areas. That pattern
corresponds to a wave at the point shown in FIG. 8.
For the wave in the above pattern, as will be seen from FIG. 40,
response to Mg and Cy is large, while response to Gr and Ye is
small. Accordingly, if the signals R, G, B are calculated in
accordance with Equation (1), a false color would be caused in
spite of an achromatic object.
Further, for the pattern as shown in FIG. 41, response to Mg and Ye
is large, while response to Gr and Cy is small, thus resulting in
the similar problem.
A thirteenth embodiment of the present invention is made to solve
the above problem and is directed to an image signal processor
which can suppress the occurrence of false colors and realize high
image quality. The image signal processor comprises image pattern
detecting means for detecting an image pattern which tends to cause
a false color, color signal processing means for making color
signal processing adapted for the image pattern, and selection
means for selecting the color signal processing means depending on
an output of the image pattern detecting means to make the color
signal processing.
The color signal processing means may carry out the process for
converting color signals into achromatic signals.
With the image signal processor thus arranged, upon detection of
the image pattern which tends to cause a false color, the color
signal processing adapted for the detected image pattern is carried
out.
The thirteenth embodiment will be described below in more detail.
FIG. 42 is a block diagram of a signal processor for a video camera
which implements the thirteenth embodiment.
A sensor 501 is associated with the color filter array shown in
FIG. 7 for reading two rows of the array in the zigzag form during
one horizontal scan period (1H) as seen from the drawing.
Accordingly, all the pixels are read out once for one field.
Assuming now that the read-out clock has frequency of fs, an output
of the sensor 501 is changed in the order of Mg, Cy, Gr, Ye for
each clock. The output signal is applied to an analog processing
unit 502 for signal processing such as AGC (automatic gain
control), and then subjected to A/D (analog-to-digital) conversion
by an A/D converter 503. The A/D converter used desirably has 10 or
more bit precision.
The resulting digital signal is inputted to an interpolation filter
504 and then to a luminance processing unit 505 for standard video
signal processing such as .gamma. conversion and blanking, followed
by D/A (digital-to-analog) conversion by a D/A converter 506.
Further, the resulting analog signal is added in a period signal
adding unit (not shown) with a synch signal for a standard TV
signal.
On the other hand, the output of the A/D converter 503 is separated
into color signals Mg, Gr, Cy, Ye and then interpolated
two-dimensionally in interpolation filters 507 to 510. This process
is described in the specification of Japanese Patent Application
No. 63-281456 filed by the present applicant, for example, and
hence will not be explained further herein. RGB conversion units
512, 513, 514 convert the four color signals Mg, Gr, Cy, Ye into
three primary color signals R, G, B in accordance with above
Equation (1) using matrix coefficient different from one
another.
A matrix selection unit 511 determines, as described later, which
one of the RGB conversion units should be used to minimize the
occurrence of false colors, and the decision result is outputted to
a switch 515. The switch 515 selects one set of RGB outputs from
the RGB conversion units 512, 513, 514 depending on the decision
result of the matrix selection unit 511.
Differences among the RGB conversion units 512, 513, 514 will be
next explained.
The RGB conversion unit 512 performs the matrix operation for to
minimize the occurrence of false colors for the bright and dark
patterns shown in FIG. 40. Specifically, for the pattern of FIG. 40
where it is assumed that Mg and Cy are bright, while Gr and Ye are
dark, R is expressed below in accordance with above Equation
(1):
As explained in the above-cited reference, too, a false color Rm of
the red color for the pattern of FIG. 40 is expressed by
multiplying +1 to the terms of Mg and Cy and -1 to the terms of Gr
and Ye as follows:
When Rm becomes zero, the occurrence of false colors is minimized.
Thus:
Since the above discussion is similarly applied to other colors,
the conditions to be met are expressed below:
A method of determining the matrix coefficients, which are also
optimized in color reproduction under the restricting conditions of
(5), is described in the above-cited Japanese Patent Application
No. 63-281456. As a result of executing the matrix operation using
the thus-determined coefficients in the RGB conversion unit 512,
the output of the unit 512 is modified such that the occurrence of
false colors is minimized for the pattern of FIG. 40.
Likewise, the conditions of minimizing the occurrence of false
colors for the pattern of FIG. 41 is given by:
As a result of executing the matrix operation, which is optimized
in color reproduction under the restricting conditions of (6), in
the RGB conversion unit 513, the output of the unit 513 is modified
such that the occurrence of false colors is minimized for the
pattern of FIG. 41.
The RGB conversion unit 514 performs the operation of converting
the color signals into achromatic signals when both the patterns of
FIGS. 40 and 41 are overlapped with each other in the color matrix.
Means for conversion into achromatic signals is implemented by
utilizing the following relationship, for example:
When both the patterns of FIGS. 40 and 41 are overlapped with each
other, this corresponds to an edge region of the object. In such a
region, the image quality will be less deteriorated even if the
color signals are converted into achromatic signals.
FIG. 43 shows the construction of the matrix selection unit 511.
The signal from the sensor 501 through zigzag reading of the pixels
is inputted to the matrix selection unit 511 while repeating the
cycle of Mg, Cy, Gr and Ye in this order. Accordingly, the result
of adding the input signal and the signal which is obtained by
delaying the input signal one data in a delay 601, in an adder 602
is given by repetition of Mg+Cy/Cy+Gr/Gr+Ye/Ye+Mg. The result of
taking the difference between the above summation signal and the
signal which is obtained by delaying the summation signal two data
in delay 603 and 604, in a subtracter 605 is given by repetition of
(Mg+Cy)-(Gr+Ye)/(Cy+Gr)-(Ye+Mg)/(Gr+Ye)-(Mg+ Cy)/(Ye+Mg)-(Cy+Gr).
If the pattern component of the object as shown in FIG. 41 is
large, an absolute value of (Cy+Gr)-(Ye+Mg) is increased. If the
pattern component of FIG. 40 is large, an absolute value of
(Mg+Cy)-(Gr+Ye) is increased. Accordingly, by changing over the
data on a data-by-data basis by a switch 606, the output is
separated such that .+-.[(Mg+Cy)-(Gr+Ye)] is led to an upper
absolute value detecting unit 607 and .+-.[(Cy+Gr)-(Ye+Mg)] is led
to a lower absolute value detecting unit 608. The absolute value
detecting units 607, 608 take absolute values of those outputs,
which are then compared with present thresholds Th1, Th2 in
comparators 609, 610, thereby producing logical outputs P, Q. P is
1 if .gtoreq.Th1 and 0 if <Th1, while Q is 1 if .gtoreq.Th2 and
0 if <Th2. A decision logic 611 determines the matrix to be used
by using the logic table shown in FIG. 44.
The conversion of the color signals into the achromatic signals is
not always required to be performed in the RGB conversion unit 514,
and may be achieved by using the so-called chroma-killer process to
make the color difference signals R-Y, B-Y zero in the color signal
processing unit 516.
Further, it is also possible to input a horizontal address X and a
vertical address Y of the pixel to the switch 515 and, if the
outputs of P and Q are zero, to switch over M1 and M2 alternately
as shown in FIG. 45.
The color filters of the sensor 501 are not limited to Mg, Cy, Gr,
Ye and may have any form, including pure such as R, G.sub.1,
G.sub.2, B, so long as four colors are arranged in the offset
structure.
In addition, this embodiment treats the two image patterns which
tend to cause false colors, and includes the three color signal
processing means 512 to 514 adapted for those image patterns.
However, this embodiment is not limited to such an arrangement and
may be implemented in any form including one or more image patterns
which tend to cause false colors, and one or more color signal
processing means adapted for the respective image patterns.
With this embodiment, as explained above, there can be obtained an
image signal processor which has high resolution and can realize
high image quality, while reducing the occurrence of false colors,
even for such an image pattern tending to cause false colors.
* * * * *